reach. The simplest solution to stopping a robot in an emergency is to
install a wireless kill switch that disables
motor drive circuitry when activated.
An inexpensive kill switch system is
available from All Electronics Corp., in
the form of a keychain remote control
shown in Figure 5. The $19 system —
which includes a pair of 27 MHz wireless key fobs — is designed to supply
12V to a remote device. However, the
main board can be reconfigured so
that the contacts of the mechanical
relay are available for direct or indirect
control of the robot motor system. I
purchased several of these units and
modified the solder pads on the boards
and remotes so that any fob can control any and all robots with a kill switch
– an especially handy feature when
working with multiple, fast-moving
robots. Line-of-sight range is about 100
feet, and about half of that indoors.
A major limitation of using a kill
switch as a remote safety device is that
it requires you to recognize something
is wrong and then press the “off”
button to deactivate a robot. An
alternative approach is to use a dead
man switch, which is roughly equivalent to a watchdog timer in that an
active signal is required to keep the
robot energized. If the onboard receiver doesn’t detect a signal because you
released a momentary-contact switch
on the remote, the receiver is out of
range, the battery is dead, or any
number of other reasons, the robot
can be configured to shut down.
Although it’s hard to beat the form
factor of the fob shown in Figure 5, the
Parallax 433 MHz transmitter and
matched receiver can be used to create
a relatively compact dead man switch.
As with most Parallax devices, setup is
quick and painless. Other than
supplying each device with 5V at 5 mA,
installation is a simple matter of
interpreting the analog received
signal strength value with an onboard
processor or using it to directly control
a solid-state relay. Line-of-sight range is
about 500 feet.
threat to eyes, ears, and extremities.
The simple act of trimming a solder
joint with a pair of diagonal cutters can
launch a wire with enough velocity to
embed it in an eye.
Unfortunately, not all construction
safety techniques can be gleaned from
common sense. If you’re unsure how
to do something, then get help.
Someone skilled in cutting and bending aluminum sheet can show you safe
techniques that could take years to
master. Given this caveat, some basics
that apply to all robot construction
area with a fire extinguisher rated for
class A (common combustibles), B
(electrical), and C (flammable liquids)
fires. An ABC extinguisher based on
Halotron is ideal for robotics work
because the hydrochlorofluorocarbon
is an electrical insulator that doesn’t
leave a residue.
• Ventilate your work area when
soldering or using adhesives. Avoid
breathing flux vapors, which can cause
permanent respiratory problems.
• Wear eye protection when using
power tools and when cutting wire,
plastic, or metal with hand tools.
• Use leadless solder and components,
and wash your hands after handling
solder. Children are especially susceptible to lead poisoning.
• Wear ear protection when working
with power tools to prevent hearing
loss. The high-pitched whine of a
Dremel can be muffled with in-ear
plugs available at most hardware
stores. In-ear plugs, while not as comfortable as over-ear protection, offer
superior sound reduction and protection from potential hearing loss.
• Use multi-strand, insulated wire for
connections that require flexibility, and
insulate exposed terminals and wires
with shrink-wrap tubing or electrical
tape. Solid wire is fine for use with
plastic prototyping boards, but more
flexible stranded wire is generally a better choice for interconnecting batteries,
sensors, power supplies, and processors – especially when there may be relative movement between components.
• Wear short sleeves when working
with power tools. Similarly, avoid loose
jewelry and anything else that might
become caught in a power tool.
• Use clamps with a drill press to avoid
a trip to the emergency room to
reattach your fingers. Clamps are
especially important when step bits are
used, because they are more likely to
bind than straight bits (See Figure 6).
• Use sealed, leak-proof batteries
whenever possible. In addition, don’t
leave batteries in your robots for
extended periods – even if they are
• Use secure construction techniques,
such as locking pins to secure parts,
instead of relying on gravity or tape.
• Use a lubricant when drilling to minimize binding and reduce noise. I like to
use a few drops of
when drilling aluminum sheeting.
Safe robot operation depends on
• Smooth freshly
cut edges with a
• Equip your work
The process of creating a robot,
whether from a kit or sheet of aluminum and plastic, poses a potential
FIGURE 6. Use a clamp
— not hands — to
stabilize items on a
drill press — especially
with a step bit.
SERVO 01.2007 55